The present invention relates to a semiconductor device, and more specifically to a semiconductor device having MOS elements and bipolar elements in a semiconductor substrate which consists of a buried layer formed between an element-forming region and a support region, and a method of forming such device.
In a semiconductor integrated circuit which consists of an MOS element, or a bipolar element, or both of them provided in an element-forming region of a semiconductor substrate, provision is made of an n-type buried layer that is used as a subcollector of an npn bipolar element; a p-type buried layer for preventing soft error which is located between a p-type well (element-forming region) of an NMOS element and a support region, which buried layer works to isolate the element from other elements and which further prevents carriers generated by alpha particles from infiltration; an n-type buried layer which is located between an n-type well (element-forming region) of a PMOS element and the support region and which works like the above-mentioned layer; or a p-type buried layer which isolates the elements between bipolar elements or between the bipolar element and the PMOS element. The buried layers must have a high concentration so that they will work favorably. From the standpoint of forming fine elements, furthermore, the buried layers must have a high concentration in compliance with the scaling rule of LSI's. In compliance with the scaling rule of LSI's, on the other hand, it becomes essential to reduce the thickness of the epitaxial layer that serves as an element-forming region. In particular, to improve characteristics of the bipolar element, the thickness of the epitaxial layer must be reduced to one micron or smaller. When a thin epitaxial layer is grown on the buried layer of a high concentration, however, impurities of the buried layer are diffused into the epitaxial layer due to the auto-doping effect, and the impurity concentration of the epitaxial layer serving as an element-forming region often changes. Such a change in the impurity concentration deteriorates the element characteristics, such as causing the threshold voltage of the MOS element to vary. If the impurity concentration of the buried layer decreases due to auto-doping, it becomes difficult to prevent the occurrence of soft error
Further, since the thickness of the buried layer increases, giving rise to the occurrence of so-called swelling of the buried layer and causing the thickness of the element-forming region to decrease, it is necessary to form a thick epitaxial layer in advance. In order to cope with such problems, there have been proposed a reduced-pressure epitaxial method, a high-temperature prebaking method in hydrogen, and a like method. These methods have been taught in Japanese Patent Publication Nos. 12010/1981 and 27757/1984.
According to the above patent publications, the auto-doping can be reduced during the epitaxial growing without, however, enabling an epitaxial film to be obtained having quality sufficient for favorably operating fine elements. Moreover, consideration has not at all been given with regard to swelling of the buried layer, wherein impurities of the buried layer transfer into the epitaxial film due to heat treatment after forming the epitaxial film, in the process for producing the elements. In particular, no effective measure has been taken to cope with the fact that the buried layer swells conspicuously due to the thermal oxidation effected at a high temperature for an extended period of time to isolate the elements.